Types of Acquired Melanocytic Nevi

Acquired melanocytic nevi are clumps of melanocytes which originate in childhood but usually manifest during the prime of life, in individuals in the 30-40 year age group. Their number increases with sun exposure and in individuals with fairer skin color. As the age increases, these nevi become more prominent, raised, or even pedunculated, but finally disappear.

Malignant change has been observed in up to a third of nevi. The risk increases with the number of nevi present such that the more nevi a person has, the greater is the risk of development of melanoma. Other risk factors include fair skin, ultraviolet radiation as with sunlight exposure, family history of melanoma, a single giant nevus, immunosuppression, and some hereditary syndromes.

Possible indicators of malignant transformation include the following, especially if there is no history of irritation:

• Rapid growth
• Irregular border
• Asymmetrical lesion
• Satellite lesion
• Different shades of color within the same nevus
• Change in the texture

For this reason, any nevus which shows alterations should be removed for histopathological evaluation. Some parents may wish any pigmented lesion to be excised because of worry.
If there is concern about changes or parental anxiety, the nevus should be removed.

Common Acquired Melanocytic Nevi (CAMN)

CAMN or “signature nevi” are common nevi. They are well-defined round to oval lesions, with regular borders. They are of three main types:

Junctional Nevi

These form macules with the skin markings becoming slightly more prominent. They are circular flat lesions, with a uniform brown coloration. The presence of a pigmentary network which fades out peripherally is characteristic. They have lentiginous cells which are round or ova, or spindle-shaped. They express the S-100 protein, as well as one or more melanoma-associated antigens such as HMB-45  or NKI/C3.

Dermal Nevi

These are raised pale brown or skin-colored lesions, and often hairy. They have focal globules and fine vessels or pale areas. They show large junctional nests of melanocytes. Pseudo-horn cysts may be seen. The cells are epithelioid with amphophilic cytoplasm, and the melanin is in granules. Deeper dermal nevi have cells with less cytoplasm, arranged in cords. Some nevi may have spindled cells similar to fibroblasts. Dermal nevus cells have only S-100 protein. This expresses maturational processes occurring in the melanocytes.

Compound Nevi

These may be raised with a wart-like surface in some cases, and are usually lighter brown than junctional nevi are. They have round or oval globules of pigment, and these may form a cobblestone pattern. Most of them show hair growth.
Most melanocytic nevi are colored but occasionally amelanotic melanocytic variants may occur.

Genetic differences

Some acquired melanocytic nevi have BRAF mutations which drive further cell signaling pathways. These are common to melanomas as well. Cell cycle arrest with beta-galactosidase activity is the result of prolonged BRAF expression. As a result, following initial proliferation, most melanocytic nevi stabilize in size by undergoing oncogene-induced senescence. Other mutations observed in these nevi include NRAS mutations.

Junctional nevi showing reticulated pigment on dermoscopy are less likely to show BRAF mutations.

Large intradermal or epidermal nevi with globules of pigment are not so significantly associated with BRAF mutations.

References

• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3965271/
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4609613/
• https://pedclerk.bsd.uchicago.edu/page/congenital-and-acquired-melanocytic-nevi
• https://www.bad.org.uk/shared/get-file.ashx?id=100&itemtype=document